ESTRO 38 Abstract book

S478 ESTRO 38

Ukraine ; 5 Albert Einstein College of Medicine, Department of Radiation Oncology, New York, USA ; 6 Peter MacCallum Cancer Centre, Department of Physical Sciences, Melbourne, Australia ; 7 University of Melbourne, Sir Peter MacCallum Cancer Institute, Melbourne, Australia Purpose or Objective In the present study, a novel technique for using a high- resolution (2 mm) 2D solid-state detector prototype ‘MP512’ in transmission mode (TM) is introduced. The technique aimed at using measurements in TM to calculate absolute dose at d max , in a homogenous phantom of solid water, in effective area (A eff ) which could be varied in size to best suit that of the monitored target. Material and Methods The MP512 has 512 diode-sensitive volumes, 0.5 mm × 0.5 mm each, uniformly distributed with a pitch of 2 mm over a square area of 52 mm side. Measurements in TM were performed with the MP512 at a surface-to-detector distance (SDD) variable in the range from 0.3 cm to 24 cm and in dose mode (DM) at d max in a homogenous phantom of solid water (Figure 1). We considered jaw-defined static square fields of 2 cm, 3 cm, 5 cm, 8 cm and 10 cm side produced by a 6 MV flattened beam delivered by a Varian Clinac® iX linear accelerator equipped with a Millennium 120-multi-leaf collimator (MLC). Measurements were used to derive a relationship between the response in DM and the response in TM as a function of SDD and field size. To verify the relationship, we calculated from measurements in TM at 4 cm and 24 cm SDD the response in DM in square fields of 1 cm and 4 cm side and in clinical step-and-shoot IMRT fields. For all IMRT plans, we used the Pinnacle’s adaptive convolution- superposition (CS) algorithm implemented into the Pinnacle 3 treatment planning system (TPS) version 14 (Philips Medical Systems, Eindhoven, The Netherlands). Dose calculations were performed at dmax with a 2 mm grid. A gamma index analysis was used to cross-check our dose calculations with TPS calculations and with measurement in DM with the MP512 itself, with Gafchromic™ EBT3 films and with a Farmer ionization chamber. Results Calculations in square fields of 1 cm and 4 cm side agreed with measured values within ±2%. Calculations in IMRT fields (Table 1) had, using acceptance criteria of 3%/3 mm, 2%/2 mm and 1%/1 mm respectively, gamma passing rates (%GP) greater than 96.89%, 90.50%, 62.20% at SDD 4 cm and greater than 97.22%, 93.80%, 59.00% at SDD 24 cm. When considering a 1%/1 mm acceptance criterion, lower %GP between our dose calculations and TPS calculations (Table 1) could be explained by factors such as sub- millimetre misalignments in detector positioning and dose averaging in TPS calculations over a 2 mm grid. This result emphasizes the importance of developing high-spatial resolution dosimetry detectors.

Conclusion We derived a relationship between the response of the MP512 in TM and in DM at d max , as a function of SDD and field size. As at a different SDD corresponds a different A eff at d max , using this relationship measurement in TM could be performed at the SSD producing the A eff which best fits the size of the monitored target. This study represented also a first step in the development of a real- time high-spatial resolution 3D dose reconstruction technique based on TM measurements with the MP512 prototype. PO-0902 The ACDS approach to measuring dose to bone and comparing to TPS reported dose to water and medium J. Lye 1 , M. Shaw 1 , J. Lehmann 2 , A. Alves 1 , R. Brown 1 , C. Davey 1 , F. Kadeer 1 , J. Kenny 1 , J. Supple 1 1 Australian Radiation Protection and Nuclear Safety Agency, Australian Clinical Dosimetry Service, Melbourne- Victoria, Australia ; 2 Calvary Mater Hospital, Physics, Newcastle, Australia Purpose or Objective In clinical dosimetry there is a difference in planned dose to a patient when dose to medium or dose to water is used. This is an issue that is particularly notable when looking at dose to bone treatments such as SABR/SBRT spine (4-12% differences depending on bone type and the definition of dose to water used by a particular TPS). It should also be considered when looking at the smaller (~1%) yet more widespread effect of using either dose to water or dose to muscle/adipose for soft tissue plans. The purpose of this work is to determine a consistent definition of dose and to apply this definition to report the differences between measurements in bone and calculated dose from TPS algorithms. Material and Methods There are three commonly used definitions of dose: Dose to medium as calculated by Monte Carlo algorithms (dose to medium-in-medium, D m,m ); Dose to water with variable electron density as calculated by “conventional TPSs” (dose to water-in-water, D w,w ); and Dose to water converted from dose to medium using stopping powers: approximates dose to the water material of a cell in an otherwise non-water medium (dose to water-in-medium, D w,m ). Of importance is that the two different definitions of dose to water are not equivalent. Our main aim is for consistent definitions, and to use only one definition of absorbed dose to water.The ACDS defines absorbed dose as the dose to medium in a medium approximated by the average J/kg across a voxel (around 1-2mm 3 ). Dose to water is then a particular case of this definition where the medium is water. Dose to water is defined as D w,w . This gives a consistent definition of dose in TPS calculations and reference dosimetry and is closest to the previously used definitions of dose which clinical data is based on. Results The ACDS measures dose to bone in the SABR spine audit with EBT3 film and microdiamond measurements in CIRS cortical and trabecular bone. These measurements are performed with a detector measuring in bone but